U.S. patent number 10,156,134 [Application Number 15/666,478] was granted by the patent office on 2018-12-18 for method and apparatus for passive detection of near-surface human-scale underground anomalies using earth field measurements.
This patent grant is currently assigned to Terra Response LLC. The grantee listed for this patent is Terra Response LLC. Invention is credited to Andrew D. Lowery, Roy S. Nutter, Franz A. Pertl, James E. Smith.
United States Patent |
10,156,134 |
Smith , et al. |
December 18, 2018 |
Method and apparatus for passive detection of near-surface
human-scale underground anomalies using earth field
measurements
Abstract
A method for detecting a subsurface anomaly at a near-surface
depth, comprises positioning an electromagnetic sensor configured
to measure a component of a planetary electromagnetic field such
that the electromagnetic sensor is suspended just above a
ground-air barrier and does not contact a ground surface; selecting
an electromagnetic frequency by calculating a function of
properties of the ground that include relative permittivity,
relative permeability, and resistivity; moving the electromagnetic
sensor over the surface of the ground; repeatedly measuring
intensity of the component of the planetary electromagnetic field
at the frequency to obtain a set of measurements; and comparing at
least a first measurement in the set of measurements to at least a
second measurement in the set of measurements to identify a change
in the intensity of the component of the planetary electromagnetic
field that is indicative of a presence of a subsurface anomaly.
Inventors: |
Smith; James E. (Bruceton
Mills, WV), Pertl; Franz A. (Morgantown, WV), Nutter; Roy
S. (Morgantown, WV), Lowery; Andrew D. (Morgantown,
WV) |
Applicant: |
Name |
City |
State |
Country |
Type |
Terra Response LLC |
Bruceton Mills |
WV |
US |
|
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Assignee: |
Terra Response LLC (Bruceton
Mills, WV)
|
Family
ID: |
51524773 |
Appl.
No.: |
15/666,478 |
Filed: |
August 1, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170356285 A1 |
Dec 14, 2017 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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14213704 |
Mar 14, 2014 |
9719343 |
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61790937 |
Mar 15, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
E21B
47/09 (20130101); G01V 3/08 (20130101); G01V
3/15 (20130101); G01V 3/12 (20130101); G01V
3/081 (20130101); G01V 3/15 (20130101); G01V
3/08 (20130101) |
Current International
Class: |
E21B
47/09 (20120101); G01V 3/08 (20060101); G01V
3/12 (20060101) |
Field of
Search: |
;324/326 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hollington; Jermele M
Assistant Examiner: McAndrew; Christopher
Attorney, Agent or Firm: McDonell Boehnen Hulbert &
Berghoff LLP
Parent Case Text
RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application
61/790,937, entitled Detection of Underground Structures Using
Earth Field Measurements, filed on Mar. 15, 2013, and incorporated
by reference as if fully rewritten herein.
Claims
What is claimed is:
1. A computer-implemented method for detecting a subsurface anomaly
at a near-surface depth, comprising the steps of: positioning an
electromagnetic sensor that is configured to measure a component of
a planetary electromagnetic field such that the electromagnetic
sensor is suspended just above a ground-air barrier and does not
contact a ground surface; selecting an electromagnetic frequency by
calculating, with a computing device, a function of properties of
the ground that include relative permittivity, relative
permeability, and resistivity; moving the electromagnetic sensor
over the surface of the ground; repeatedly measuring intensity of
the component of the planetary electromagnetic field at the
frequency to obtain a set of measurements; and comparing, with the
computing device, at least a first measurement in the set of
measurements to at least a second measurement in the set of
measurements to identify a change in the intensity of the component
of the planetary electromagnetic field that is indicative of a
presence of a subsurface anomaly.
2. The method of claim 1, wherein the selected frequency is at
least 5 kilohertz (kHz).
3. The method of claim 2, wherein the subsurface anomaly is
human-scale.
4. A computer-implemented method for determining depth of a
subsurface anomaly at a near-surface depth, comprising the steps
of: positioning an electromagnetic sensor that is configured to
measure a component of a planetary electromagnetic field such that
the electromagnetic sensor is suspended just above a ground-air
barrier and does not contact a ground surface; selecting a first
electromagnetic frequency by calculating, with a computing device,
a function of properties of the ground that include relative
permittivity, relative permeability, and resistivity; selecting a
second electromagnetic frequency by calculating, with a computing
device, a function of properties of the ground that include
relative permittivity, relative permeability, and resistivity;
moving the electromagnetic sensor over the surface of the ground;
repeatedly measuring intensity of the component of the planetary
electromagnetic field at each of the first frequency and the second
frequency to obtain a set of measurements; determining, with the
computing device, which of the first electromagnetic frequency and
the second electromagnetic frequency exhibits a greater change in
intensity attributable to the subsurface anomaly; and calculating,
with the computing device, a depth of the subsurface anomaly;
wherein calculating the depth is based at least in part on measured
intensity, at the electromagnetic frequency determined to exhibit
the greater change in intensity, of the component of the planetary
field.
5. The method of claim 4, wherein the first electromagnetic
frequency and the second electromagnetic frequency are each at
least 5 kHz.
6. The method of claim 5, wherein the subsurface anomaly is
human-scale.
Description
FIELD
This application relates to passive detection of human-scale
underground structures at near-surface depths using earth field
measurements.
BACKGROUND
It is often desirable to sense the location of subsurface objects
from outside of the surface of the material in which it is encased
(i.e. an object buried underground). For example, sensing the
presence of human-scale subsurface objects (both metallic and
non-metallic) at relatively near-surface depths (i.e., between zero
and 30 meters) can save time, costly explorative excavation, and
avoid possible damage to subsurface objects through unguided
excavation. Dangers related to digging up objects, such as
explosive land mines or gas utility lines, do not have to be
contended with or can be reduced if remote sensing from the surface
locates the object prior to excavation.
A number of methods have been developed to locate subsurface
objects. Subsurface objects, which can be referred to as anomalies,
may have various compositions and also include an air pocket or any
void or volume uniquely different than the surrounding homogeneous
or predictably non-homogeneous material. Metallic objects can be
found relatively easily with devices such as metal detectors and
through a host of other technologies, such as Ground Penetrating
Radar (GPR). It is, however, much more challenging to find
non-metallic subsurface objects. The invention described herein is
a passive method and apparatus for detecting both metallic and
non-metallic subsurface objects, voids and other anomalies using
the natural electromagnetic signal emanating from Earth's
interior.
The Earth's interior is a highly dynamic structure comprised of
multiple layers with a fluid behavior. As the Earth rotates,
portions of this fluid move at different velocities and directions.
This motion (as well as other factors including lightning, solar
wind and flares, etc.) generate low level electromagnetic signals,
which then travel outward and pass through the Earth's surface. One
example of this phenomena is the well-known core-dynamo effect that
creates the quasi-steady state geomagnetic field within the planet.
Heating, conduction, and swirling of molten rock can also produce
mechanical and electrical signals that travel towards the surface.
As these signals travel towards the Earth's surface, they will be
affected by the material through which they travel. This effect may
show up as variations in signal strength, signal phase, frequency,
etc. As the composition of the material varies, so will its effects
on the signal passing through it. By monitoring, over an area, the
signals emanating from below the Earth's surface, material
variations can be detected. This effect can be employed and adapted
to locating subsurface objects, voids or other anomalies.
One method of detecting underground structures and other anomalies
is audio magneto tellurics (AMT), which monitors AC-signals in the
audio frequency range to discover extremely large-scale geological
structures. These structures, referred to herein as being of
geologic scale, include, by way of example, layers of mineral
deposits, rock formations, or other natural resources (such as, for
example, coal seams). AMT and other known techniques may not be
effective for detecting subsurface objects on smaller scales, at
higher resolutions, or at shallower depths.
Another method for detecting underground structures and other
anomalies is passive magneto tellurics, which relies on natural,
lightening-driven atmospheric noise signals, such as lightening and
magnetosphere activities. U.S. Pat. Nos. 4,507,611, 4,825,165 and
5,148,110 to Helms, et al., which are incorporated herein by
reference in their entireties, disclose such and other methods for
detecting subsurface anomalies. U.S. Pat. No. 6,414,492 to Myers,
describes another method for detecting geophysical discontinuities
in the Earth by measuring the electrical component of the Earth's
electromagnetic field at frequencies below 5 kHz.
These identified methods are capable, to varying degrees, of
detecting large, or geologic-scale anomalies at significant
sub-surface depths. For example, passive magneto tellurics can
detect geological-scale anomalies starting at depths from a few
tens of meters to many kilometers, but lacks the resolution to
detect small, human-scale objects. Similarly, the passive method
disclosed in U.S. Patent No. 5,414,492 can detect geologic-scale
anomalies at depths greater than 22.5 meters. The identified
methods are not, however, capable of detecting human-scale
anomalies or detecting both metallic and non-metallic anomalies at
more shallow, near-surface depths (i.e., between zero and 30
meters). For example, none of these methods is sufficiently capable
of detecting human-scale anomalies, such as plastic pipes, storage
tanks, land mines, or other man-made objects (referred to herein as
human-scale objects), buried at near-surface depths. Moreover, the
identified methods are capable of generating only relatively
low-resolution representations or images of detected subsurface
anomalies and have limited capability for determining
characteristics of detected subsurface anomalies, such as
composition.
Thus, there exists a need in the art for methods and apparatus to
passively detect human-scale anomalies, to detect both metallic and
non-metallic anomalies, to detect anomalies at near-surface depths,
to provide higher resolution representations or images of detected
subsurface anomalies, and to determine characteristics of detected
subsurface anomalies, such as composition, than what presently is
known or available in the art.
SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts in a
simplified form that are further described below in the Detailed
Description. This Summary is not intended to identify key or
essential features of the claimed subject matter, nor is it
intended to limit the scope of the claimed subject matter.
In one embodiment, a method for detecting a human-scale, subsurface
anomaly at near-surface depths below an area of the Earth's surface
comprises suspending a sensor for measuring a component of the
Earth's electromagnetic field proximate to the ground-air barrier,
measuring the intensity of a component of said field over the area
at a frequency of 5 kHz or greater, and comparing the measurements
to identify variations in the intensity of the field within the
area to detect the presence of a human-scale, subsurface anomaly at
a near-surface depth below the first area.
In one embodiment, the presence of an anomaly is detected by
comparing measurements of the electric component of the Earth's
electromagnetic field at a frequency of 5 kHz or greater. In
another embodiment, the presence of an anomaly is detected by
comparing measurements of the magnetic component of the Earth's
electromagnetic field at a frequency of 5 kHz or greater. The
presence of an anomaly also may be detected by comparing
measurements of the electric and magnetic components of the Earth's
electromagnetic field at a frequency of 5 kHz or greater. In one
embodiment, the method may include using variations in an
electromagnetic property to determine characteristics of the
detected anomaly. Variations in an electromagnetic property may be
detected using an array of sensors that may comprise a plurality of
sensors, and the location of each measurement may be determined, at
least in part, using triangulation.
In one embodiment, a method for determining the depth of a
human-scale, subsurface anomaly at near-surface depths below a
first area of the Earth's surface comprises suspending a sensor for
measuring a component of the Earth's electromagnetic field
proximate to the ground-air barrier within the area, measuring the
intensity of a component of said field over the area at a plurality
of frequencies of 5 kHz or greater, identifying the frequency
demonstrating the greatest change in intensity in the presences of
the anomaly, calculating the depth of the anomaly using the
identified frequency, and determining one or more characteristic of
the composition or makeup of said anomaly with a most likely
material based on a host of possible materials.
In one embodiment, the depth of an anomaly is determined by
measuring the intensity of the electric component of the Earth's
electromagnetic field at a plurality of frequencies of 5 kHz or
greater. In another embodiment, the depth of an anomaly is
determined by measuring the intensity of the magnetic component of
the Earth's electromagnetic field at a plurality of frequencies of
5 kHz or greater. In yet another embodiment, the depth of an
anomaly is determined by measuring the intensity of the electric
and magnetic components of the Earth's electromagnetic field at a
plurality of frequencies of 5 kHz or greater. In one embodiment,
the method may include using variations in an electromagnetic
property to determine characteristics, such as size, shape and
material composition, of the detected anomaly. Variations in an
electromagnetic property may be detected using an array of sensors
that may comprise a plurality of sensors, and the location of each
measurement may be determined, at least in part, using
triangulation.
Another embodiment is an apparatus for detecting human-scale
objects below the surface of the Earth at near-surface depths that
comprises a sensor for measuring a component of the Earth's
electromagnetic field at frequencies greater than 5 kHz, a
frequency-selective circuit, an amplifier, and a recording device.
In one embodiment, the sensor is capable of measuring the electric
component of the Earth's electromagnetic field at a frequency of 5
kHz or greater, and in another embodiment, the sensor is capable of
measuring the magnetic component of the Earth's electromagnetic
field at a frequency of 5 kHz or greater. In an alternate
embodiment, the sensor is capable of measuring the electric and
magnetic components of the Earth's electromagnetic field at a
frequency of 5 kHz or greater. One embodiment further includes a
receiver for determining the location of each measurement using
triangulation, or other means for determining position.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the typical electric and magnetic field
radiating from the Earth.
FIG. 2 illustrate the difference in intensity versus position for
typical electromagnetic fields radiating from the Earth.
FIG. 3 illustrates an example of distortions of the Earth's
electromagnetic field.
FIG. 4 illustrates a second example of distortions of the Earth's
electromagnetic field.
FIG. 5 illustrates how an object located below the air-ground
barrier can distort components of the Earth's electromagnetic
field.
FIG. 6 illustrates the effects of an object located below the
air-ground barrier on the electric component of the Earth's
electromagnetic field at varying frequencies.
FIG. 7 illustrates the typical electric and magnetic fields
radiating from the Earth in the presence of a non-magnetic
anomaly.
FIG. 8 illustrates the differences in intensity versus position for
the electric and magnetic components of the typical electromagnetic
field radiating from the Earth in the presence of a non-magnetic
anomaly.
FIG. 9 illustrates the typical electric and magnetic fields
radiating from the Earth in the presence of a magnetic anomaly.
FIG. 10 illustrates the differences in intensity versus position
for the electric and magnetic components of the typical
electromagnetic field radiating from the Earth in the presence of a
magnetic anomaly.
FIG. 11 illustrates distortions in the electric field caused by an
object with a strong dielectric constant.
FIG. 12 illustrates an exemplary sensor configuration.
FIG. 13 illustrates an exemplary circuit for detecting the presence
of a human-scale anomaly at a near-surface depth.
FIG. 14 illustrates a field mill sensor for measuring components of
the Earth's electromagnetic field.
DETAILED DESCRIPTION OF THE INVENTION
Reference will now be made in detail to exemplary embodiments of
the present invention, examples of which are illustrated in the
accompanying figures. Other embodiments may be utilized and
structural and functional changes may be made without departing
from the respective scope of the invention. Moreover, features of
the various embodiments may be combined or altered without
departing from the scope of the invention. As such, the following
description is presented by way of illustration only and should not
limit in any way the various alternatives and modifications that
may be made to the illustrated embodiments and still be within the
spirit and scope of the invention.
Because of its properties of an electromagnetic resonator, the
Earth has time-varying electric and magnetic fields. As shown in
FIG. 1, the Earth's electromagnetic field 100 includes electric
field component 110 and magnetic field component 120, both of which
has three components, one in the x-, the y-, and the z-directions.
Typically, all three components will travel through the Earth, and
penetrate the ground-air barrier 130. However, because electric
fields tend to radiate radially from the Earth (as with every
spherical object), the z-component of electric field 110 will
ordinarily be the strongest by orders of magnitude.
On a clear day, the Earth's electric field has an approximate
strength of 100 V/m, and its magnetic field has an approximate
strength of 0.25 to 0.65 gauss. Thus, given a network of sensors
located just about the surface of the Earth, the response of the
electric and magnetic fields should be essentially constant over a
given area of the Earth's surface, as illustrated in FIG. 2 for
electrical field E and magnetic field M (both at all frequencies
within the range). These intensities may vary based on the
frequency being measured, with lower (fundamental) frequencies
typically having higher intensities and lower (harmonic)
frequencies having lower intensities. The existence of subsurface
objects and other anomalies will cause distortions in these
electromagnetic fields due to differences in the electromagnetic
properties of the subsurface object and its surroundings.
Accordingly, both the electric and the magnetic components of the
Earth's electromagnetic field can convey information that is useful
for passively detecting relatively smaller scale (i.e.,
human-scale) subsurface objects and anomalies at relatively
near-surface depths (i.e., less than 30 meters).
Electromagnetic waves abide by the same properties as other waves
in nature. These include superposition and elimination,
attenuation, as well as a host of others. Some properties, such as
attenuation when traveling through mediums, are frequency based.
The concept is known as skin effect, and can be described by
Equation 1, as follows:
.delta..times..times..rho..omega..times..times..mu..times..rho..times..ti-
mes..omega..times..times.
.rho..times..times..omega..times..times..times..times. ##EQU00001##
where .delta. is the skin depth, .rho. is the resistivity, .omega.
is the angular frequency (2.pi.*frequency of operation), is the
total permittivity, and .mu. is the total permeability of the
material. By measuring electromagnetic signals emanating from the
Earth at closely spaced locations just above the ground, it is
possible to determine if and where a subsurface object, void, or
other anomaly exists. The signal modification, be it by attenuation
in magnitude or some other electromagnetic property, is most
pronounced in close proximity to and directly above a subsurface
object. This is a result of the general vertical direction in which
the signal propagates and the refractive nature of the ground-air
boundary at relatively low frequencies.
Two examples of distortions of the electromagnetic field are
illustrated in FIGS. 3 and 4. The source of the electromagnetic
fields being distorted can be varied in origin. The electromagnetic
environment may be composed of any number of varying field
components, including but not limited to natural phenomena such as
lighting strikes, the quasi-static earth-to-ionosphere potential
and many other internal effects or man-made signals, such as those
that are emanating from electric power lines, or electromagnetic
fields deliberately created in the vicinity of the object with a
signal generator, that could be used for sensing purposes. The
distortions in these fields will be functions of frequency and
location. The nature of the distortions will be foremost
characterized by the differences in material properties between the
subsurface object and its surroundings. The most influential of
these properties include conductivity, dielectric permittivity, and
magnetic permeability. These properties are, in turn, potentially
affected by other properties, such as material porosity and
moisture content. An apparatus with at least one, but preferably
many, field probe(s) can sense such distortions in said fields.
Suitable electromagnetic field probes can include transducers, such
as electric field probes or magnetic pickup coils, among many other
options. The signals sensed over an area may be recorded and
processed with a computational device to estimate the location and
nature of the subsurface object. In order for said field
distortions to be measurable, the sensing probe should be in close
proximity to the surface, and the object may not be located too
deeply or too distant from the probes, and abide by good practice
in collecting and recording electronic signals. Note that frequency
dependent conductivity of the surrounding material may act as a
sufficient shield and suppress distortions at certain
frequencies.
FIG. 5 illustrates this general idea. Object 500 is located
relatively near the Earth's surface, denoted by ground-air boundary
510 and in the field of composite signal 520 emanating from within
the Earth. The presence of object 500 results in attenuation of
composite signal 520 that is detectable above the surface of the
Earth in area 530. Possible refraction of signal 520 about object
500 may create more complicated effects than the simple "shadowing"
illustrated. A suitable sensor can be employed as described herein
to detect the alteration in composite signal 520 resulting from the
presence of object 500. The alterations will contrast measurements
in regions where composite signal 520 has not been affected by the
presence of object 500.
Based on the effective skin depth, or the depth of an anomaly, both
the electric and magnetic field strength intensities can be
detected by stimulating a range of frequencies. As illustrated in
FIG. 6 with respect to the Earth's electric field, object 600 is
located below the Earth's surface and within its electric field,
shown as flux field 610. Measuring the intensity of electric field
610 for a range of frequencies f.sub.1, f.sub.2 and f.sub.3 over a
given distance or area, as shown in FIG. 6, the existence of object
600 causes a change in intensity for each of frequencies f.sub.1,
f.sub.2 and f.sub.3. The detectable changes in intensity are
indicative of the presence of object 600. Moreover, because the
change is greatest for frequency f.sub.2, the depth of object 600
can be determined according to Equation 1, above. Thus, both the
existence of object 600 and its depth can be ascertained by
measuring the change in the intensity of electric field 610 over a
distance. The same process also can be used to detect objects by
measuring changes in the intensity of the Earth's naturally
occurring magnetic field. Similarly, by knowing the distance above
the surface traversed by the sensor probe and the depth of the
object will provide s relative dimension for the anomaly.
When a subsurface anomaly is present in the Earth, either or both
of the electric and magnetic field components can change based on
the material characteristics of the object. Consider, for example,
non-magnetic anomaly 700 having a relative permittivity .sub.r and
relative permeability .mu..sub.r that differ from the permittivity
.sub.e and permeability .mu..sub.e of the Earth in the vicinity of
the anomaly, as shown in FIG. 7. When electric component 710a of
the Earth's electromagnetic field passes through non-magnetic
object 700, the intensity of the electric field will change in the
vicinity of the anomaly, usually as an increase in electric field
strength. As discussed above, this change in electric potential E
of electromagnetic field 710 can be measured over an area or
distance, as shown in FIG. 8 for frequencies f.sub.1, f.sub.2 and
f.sub.3, with frequency f.sub.3 having the closest relationship to
the depth of non-magnetic object 700 below ground-air barrier 720.
However, because there is no perturbation of magnetic component
710b of the Earth's electromagnetic field, there will be no
measurable change in the intensity of magnetic field M as shown in
FIG. 8 due to the presence of non-magnetic object 700.
As a further example, the presence of a magnetic (paramagnetic to
ferromagnetic in classification) anomaly with relative permittivity
more than 1 and permeability more than 1 will cause variation with
respect to both the electrical and magnetic components of the
Earth's magnetic field. FIG. 9 shows the presence of magnetic
object 900 having a relative permittivity .sub.r and relative
permeability .mu..sub.r that differ from the permittivity .sub.e
and permeability .mu..sub.e of the Earth in the vicinity of the
anomaly within electromagnetic field 910. When electric field
component 910 a passes through magnetic object 900, the intensity
of the electric field will change in the vicinity of the anomaly
(again, usually, this is an increase in electric field strength).
Additionally, the intensity of magnetic field component 910b of
electromagnetic field 910 will change in the vicinity of the
anomaly, with the change in magnetic potential increasing or
decreasing based on the orientation of magnetic object 900
vis-a-vis magnetic component 910b of electromagnetic field 910. As
shown in FIG. 10, both electric potential E and magnetic potential
M change in intensity in proximity to magnetic object 900 at
frequencies f.sub.1, f.sub.2 and f.sub.3. Because the electrical
field radiates radially, it remains the most useful for determining
the depth of magnetic object 900. The depth of magnetic object 900
below ground-air barrier 920 thus can be ascertained based on
frequency f.sub.3 since it demonstrates the greatest change in
intensity. Once the depth of the anomaly is discerned, the
remaining electric and magnetic characteristics can be used to
determine the most likely material composition (given a lookup
table, or other suitable means for comparison, is available). Thus,
by measuring the changes in both the electric and magnetic fields,
a more accurate characterization can be performed regarding the
composition, size and material structure of the buried subsurface
anomaly.
In one embodiment, a subsurface object may be detected through the
distortions in the electric field caused by differences in
properties of the subsurface object and its surroundings. An
example of this type of distortions resulting from an object with a
strong dielectric constant in a static electric field is
illustrated in FIG. 11. This figure shows the electric field lines
bending toward the region in space with higher dielectric property.
In detecting subsurface objects, the static field lines may bend
toward or away from the object depending on the material of the
object compared to the surroundings. The nature and severity of the
distortion will be foremost characterized by the differences in
material properties between the subsurface object and its
surroundings. Consequently, a passive detection method such as this
one could be used to locate anomalies on top of each other (with
some earth between them) as well. The material properties that are
very influential on a static electric field include conductivity,
and dielectric permittivity. These properties may, in turn, be
affected by other properties, such as material porosity and
moisture content.
An apparatus with at least one, but possibly many, sensors can
detect and measure distortions in one or more components the
electromagnetic field as a function of position. The position of
each measurement may be determined by using a network of known
positions with a triangulation scheme, including global positioning
system, or other appropriate means. The signals sensed at various
locations over an area and the location at which a signal is sensed
may be recorded and processed with an appropriately-configured
computational device to estimate the location and nature of the
subsurface object.
For the field distortions to be measurable and to minimize the
effects of potential sources of electromagnetic and other forms of
interference, the sensing probe should be in close proximity to the
surface. This is because the density of the field's flux lines tend
to re-equalize at large relative distances from a given object and
thus the distortions may no longer be detectable if the probes are
far away. In one embodiment, non-geological scale objects having
relative permittivity and permeability that differ from the
permittivity and permeability of the Earth in the vicinity of the
anomaly can be detected at near-surface depths at frequencies
greater than 5 kHz, with the preferable frequencies being a
function of relative permittivity, which can range from .sub.r=1 to
100,000, relative permeability, which can range from .mu..sub.r=1
to 1,000,000, and the resistivity .rho. of the Earth, which
typically is in the range of 10 to 1,000 Ohm-m and can vary up to
10 times higher and lower in extreme situations.
In one embodiment, the signal is electromagnetic in nature and is
monitored over an area by several sensors. Monitoring the signal
over a suitably sized area can be accomplished by mechanically
scanning sensors over the area, or by having a multitude of sensors
distributed in some suitable fashion over the region. FIG. 12 shows
an exemplary sensor configuration 1200 comprising nine sensors
1210. Each sensor, or network of sensors, may include a suitable
arrangement of electronics and transducers, such as electric field
probes or magnetic pickup coils with appropriate amplification and
conditioning electronics. Sensors also may include permalloy
sensors, including sheets of such sensors. Other sensors capable of
sensing electrical or magnetic fields with sufficient sensitivity
may be used, as would be understood to one of ordinary skill in the
art. Such sensors could be in a handheld device (i.e. a scanner or
metal detector sized apparatus), attached to a rotorcraft (such as
RC helicopters or quadcopters), or another portable apparatus.
The sensor signals are preferably recorded and processed by a
computational device to extract the location of the subsurface
object. The recording and processing is preferable done with a
suitable computer and data acquisition system. The processing may
involve computing differences in sensor responses as functions of
sensor location, time the signal is measured, and frequency of the
signal. In one embodiment, the processing only concentrates on an
electromagnetic signal of low audio and sub-audio frequencies,
since attenuation of electromagnetic signal strength during
propagation rises with increased frequency. Results of the
computation then may be displayed in some fashion so the subsurface
object can be located. The display could be as complex as a two- or
three-dimensional map of the area scanned or as simple as an
indicator light that activates on the sensing apparatus when over
the object.
In one embodiment, the sensor apparatus includes an array of
probes, arranged spatially over an area in which distortions are to
be measured. This arrangement may include a rectangular grid of
probes, a hexagonal tiling, or some other regular or irregular
arrangement. The probes themselves can be embodied by a plate,
field sensing dipoles, coils, or other structures composed of a
suitable material. The probes should be suspended in close
proximity to the ground, and be oriented to measure at least the
vertical electric field component. This field component should be
particularly strong due to the relatively high conductivity of the
ground compared to the air above. (Electric field lines orient
themselves to impinge normally on "good" conductors.) If coils are
employed, horizontal magnetic field components may be of greater
interest. Other field components may be measured, as well, and
additional information may be gained from measuring such field
components.
The suspension of the probes in close proximity to the ground
should be such as to not interfere with or obscure the field to be
measured. The suspension framework thus should preferably be
constructed of a material that has electrical properties as close
to air as possible. For example, closed-cell extruded polystyrene
foam and other hardened foamed materials have been found to possess
reasonably suitable qualities.
As shown in FIG. 13, the signal collected from each of probes
1300a-c may be passed through its own frequency selective circuit
1310a-c to reject signals not of interest prior to amplification of
such signals by amplifiers 1320a-c. In one embodiment, frequency
selective circuit 1310 a-c may include low noise band pass filters
or selectively variable filters to prevent overloading of
amplifiers 1320a-c by excessively strong undesired signals. After
undesired signals are filtered or otherwise removed or otherwise
addressed, each probe signal is amplified by amplifiers 1320a-c.
Each probe signal preferably is amplified by its own high impedance
instrumentation grade amplifier to recordable levels. The amplified
signals are then transferred to a recording device 1330, where they
can be stored, analyzed and interpreted. In one embodiment, the
signal is digitized either before or after being transmitted or
communicated to recording device 1330 and analyzed on a digital
computer.
In one embodiment, each probe, frequency selective circuit, and
amplifier is a small self-contained unit that links to the
recording device, such as a computer, by a method that will not
cause field distortions. A probe may include a small dipole
antenna, an integrated circuit chip that performs frequency band
selection and amplification, a miniature battery, and a fiberoptic
interface that carries the amplified signal to the data recording
device that has been prepared in such a manner as not to interfere
with the signal to be measured. For example, the device could
include of an electromagnetic interference (EMI) shielded laptop
computer. Shielding may be accomplished by placing the recording
device in a steel box at some distance from the sensing probes or
in other ways that will be appreciated by those of ordinary skill
in the art.
In one embodiment, all probe signals are recorded simultaneously
(in parallel) to remove the time varing randomness in the
electromagnetic fields used for sensing. The signals are then
processed, preferably by a computer with appropriate software, or
some alternate mechanism. The processing may include calculating
one or more metrics to extract signal differences from
probe-to-probe. Calculating a metric could, for example, include
decomposing the recorded signals into their frequency components
(spectral analysis) and then comparing probe to probe variations at
various frequencies. If multiple field components are measured,
techniques such as principal component analysis could also be used
to determine the orientations of the largest field differences.
Comparing probes, spaced farther apart in the array, as opposed to
probes located adjacent, may allow relative depth probing. As
discussed above with respect to Equation 1, a relative correlation
exists between depth and frequency, since electromagnetic field
penetration into a conductive medium will decrease with increasing
frequency (i.e., the skin effect).
In an embodiment, the sensor apparatus includes a stationary
reference static electric field sensor, and one or more static
field sensors that can mechanically scan over an area. The
stationary field sensor's signal is used as a reference, to compare
to other sensor readings. The mechanical scanning may be
accomplished with a suitable x-y scanning mechanism. A field sensor
may include a device such as an electric field mill as shown in
FIG. 14, a field effect transistor with an appropriate probe, or
some other device that can properly sense the presence and
orientation of a static electric field. In FIG. 8, the field mill
measures electric field 1400 using fixed electrodes 1410 that are
alternately shielded and exposed to field 1400 by spinning rotor
1420, resulting in a modulation of induced electrical charge.
Charge amplifier 1430 can then convert the modulated charge into
voltages proportional to the strength of field 1400 that can then
be measured by volt meter 1440. The electric field sensors should
be suspended in close proximity to the ground, and in one
embodiment, are oriented to measure the vertical electric field.
The electric field on the surface of the Earth is oriented
vertically due to the relatively high conductivity of the ground
with respect to the air directly above, as electric field lines
orient themselves to impinge normally on "good" conductors. The
field thus may be thought of as a charged parallel plate capacitor,
where the Earth and the ionosphere comprise the plates of the
capacitor, and the electric field exists between the two. Mounting
the sensors close above the ground should be such as to not
interfere with the measurement of the static electric field.
Once the electric field component or other signals have been
recorded as, for example, a function of position over the area as,
for example, a digitized signal on a computer or microprocessor,
they can be analyzed by suitable means to reveal distortions that
indicate subsurface objects, voids, or other anomalies. The results
can then be displayed. In one embodiment, the results may be
displayed as 2- or 3-dimensional estimations of a subsurface object
or its location, or as simple as an indicator light signaling the
presence of a subsurface object. Recognition and classification
techniques may be employed to further improve the usefulness of the
results for a given objective. For example, anomolies that meet
certain criteria may be indicated in a manner that is different
from other areas, such as highlighting an area with characteristics
consistent with a possible landmine or underground utility pipe,
where as other subsurface objects such as rocks may be ignored or
shown in other colors or representations.
Various embodiments of the invention have been described above.
Modifications, alterations, and/or combinations of the embodiments
presented will occur to others upon the reading and understanding
of this specification. The claims as follows are intended to
include all modifications, alterations, and/or combinations insofar
as they come within the scope of the claims or the equivalents
thereof.
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